Why Do We Drive 55mph?

by | May 9, 2012 | Economy

Ever wonder why the speed limit is what it is? Why 55 mph? Why not 60? Or something random, like 48?
Well, it has to do with Congress and the first OPEC (Organization of Petroleum Exporting Countries) gas embargo during the early 1970s. Congress passed a law mandating 55 mph as the national speed limit. But what were they thinking? How did they come up with this number?
Actually, fifty-five miles per hour is really not the most fuel-efficient speed. You get the best fuel economy when your torque converter first goes into lock-up in top gear; in a traditional automatic, four cylinder engine vehicle, that’s usually around 40 mph and sometimes as low as 25 mph.
So, why 55?
Well, it has to do with wind resistance, actually. It turns out that when it comes to wind resistance, at 30 mph wind resistance is four times as great compared to when a car is going 30 mph. When wind resistance hits 55 mph, the curve begins to rise almost vertically, so at 65 mph, wind resistance is greater than at 55 mph.
If that doesn’t seem like it’s that much of a difference, it actually is. Here’s an explanation that might be easier for some of us to understand: Imagine walking in a waist-deep swimming pool. It’s not hard to do if you walk slowly, but trying to walk at your normal walking speed is much harder, and trying to run is practically impossible!
Another reason the national speed limit was set at 55 mph and not 45 or 50 mph: time. For a fairly short trip of about 20 miles, the time penalty for driving at 55 mph versus 65 mph is just about three minutes; however, the fuel consumption penalty for driving faster averages nearly 20%, but for some vehicles it may be much higher.
So, next time you’re on the highway and frustrated that the 55 mph speed limit is just too slow for you, think about the gas you’re saving, and remember, you’re only going to get to your final destination a couple minutes quicker by speeding up to or 15 mph!
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